Along with recent power increase of engines, the requirements for piston rings have become more and more severe. Improved wear resistance and scuffing resistance are required for piston rings. With a view to reducing cost by means of saving oil consumption and decreasing the running-in time, the so-called initial running-in property must be improved, that is, early running-in of the piston rings and a cylinder liner should be attained.
Heretofore, in order to improve the initial running-in properties of the sliding members, for example piston rings, a copper plating film or a thermal spray soft coating film has been formed on the outermost peripheral surface of the piston rings. However, when the piston rings provided with the copper plating film are used in a high-performance Diesel engine, since the copper plating film is too soft, it wears off before the initial running-in is completed, which disadvantageously leads to wear and scuffing of the piston rings.
It is disclosed in Japanese Published Patent Application (kokai) No. 11-80921 that a copper-based alloy sliding material, consisting of from 7 to 11 mass % of Al, from 0.5 to 8 mass % of one or more of Fe, Ni and Mn, and the balance of Cu, is plasma or arc sprayed to form a thermal spray coating film, to be used as an initial running-in coating film. However, since this coating film is hard and is free of a self-lubricating component, the initial running-in property is not satisfactory.
Furthermore, it is disclosed in Japanese Translation of PCT International Application No. 2000-507679 that the thermal spray coating film, which contains from 5 to 60% of a solid state lubricant, such as graphite, from 40 to 95% of a metal binder such as Ni, Al and Si, and optionally less than 15% of an organic binder, is used to solve the problems as described above. However, since this coating film is as soft as the Cu plating film, the wear resistance is unsatisfactory such that the coating film disadvantageously wears off before the initial running-in is completed.